1. Field of the Invention
The present invention relates to an autofocus camera.
2. Related Background Art
As is well known, an autofocus camera receives an object image formed by a focus detection optical system using a charge accumulation type sensor, detects a defocus amount of the object image surface with respect to a prospective focal plane of a taking optical system by calculating the sensor output, and drives a focusing lens in accordance with the detected defocus amount, thereby achieving an in-focus state of the taking optical system.
When an autofocus (to be abbreviated as AF hereinafter) camera of this type executes a continuous shooting operation, if the charge accumulation time of the charge accumulation type sensor between each two adjacent frames is prolonged, a high-speed continuous shooting operation is disabled. For this reason, a technique for shortening the accumulation time by increasing the amplification gain of the sensor output in the continuous shooting mode is disclosed in Japanese Laid-Open Patent Application No. 2-64517.
However, when the accumulation time is shortened by merely increasing the amplification gain of the sensor output, since the charge accumulation amount itself before amplification decreases, the (S/N) sitnal-to-ratio of the sensor output consequently decreases, and detection precision of the defocus amount deteriorates. When a continuous shooting operation is executed while executing AF control in accordance with this defocus amount, photographs in an out-of-focus state are often obtained.
Also, a multi AF area AF camera is known. This camera comprises a plurality of sets of focus detection optical systems and sensors, detects a plurality of defocus amounts by executing focus detection for a plurality of areas set on a taking field, determines a single final defocus amount based on these defocus amounts, and drives a focusing lens in accordance with the final defocus amount, thereby achieving an in-focus state of a taking optical system.
However, in the above-mentioned multi AF area AF camera, a time required for AF control (to be referred to as an AF time hereinafter), including the charge accumulation time, the charge transfer time, and the focus detection time of the sensors, increases since the number of AF areas increases, as compared to a single AF area AF camera which performs focus detection using a single area, resulting in a long focusing response time.
In particular, when a continuous shooting operation is performed using the multi AF area AF camera, the AF time between each two adjacent frames increases, and the frame speed cannot be increased.
FIG. 23 is a timing chart of an AF sequence for explaining the above-mentioned problem. For the sake of easy understanding, assuming that focus detection is performed while operating first and second sensors corresponding to two AF areas, an AF time Ta is given by: EQU Ta=MAX(I1, I2)+F1+F2+A1+A2+L, (1)
where I1 and I2 are the charge accumulation times of the first and second sensors, F1 and F2 are the output transfer times of the first and second sensors, A1 and A2 are the output calculation times of the first and second sensors, and L is the lens drive time.
Furthermore, the following drive system (to be referred to as a predictive drive system hereinafter) is known. That is, whether or not an object is moving is discriminated on the basis of a plurality of defocus amounts which are time-serially generated, and when it is determined that the object is moving, a lens is driven by correcting a normal lens drive amount, thereby following the moving object and maintaining an in-focus state.
The basic principle of object movement discrimination in the above-mentioned predictive drive system will be described below with reference to FIG. 35.
Referring to FIG. 35, the position of the image plane is plotted along the ordinate, and the time is plotted along the abscissa. A solid curve represents the locus of the object image surface when a taking optical system is ideally driven to follow a moving object, and a broken curve represents the position of the image surface corresponding to an actual lens position. Therefore, the difference between the solid curve and the broken curve represents the difference between the positions of the image surface=the defocus amount. If the defocus amount at time t1 is represented by d1, the defocus amount at time t2 is represented by d2, the image surface moving amount upon lens drive control between times t1 and t2 is represented by L12, and a moving speed v12 of the image surface upon object movement is given by: EQU v12=(d2+L12-d1)/(t2-t1). (2)
When the absolute value of the image surface moving speed v12 is equal to or larger than a predetermined value, it can be determined that an object is moving.
When it is determined that the object is moving, lens drive control can be executed in consideration of movement of the image surface upon object movement, so that the solid curve and the broken curve coincide with each other at time t3, and the defocus amount becomes zero.
The lens drive amount at this time can be determined in correspondence with a corrected defocus amount d2' obtained by correcting the defocus amount d2 in accordance with the following equation (3): EQU d2'=d2+v12.times.(t3-t2). (3)
In the above-mentioned system, since the moving speed of the object image surface is calculated from the defocus amounts between two times, the response time to a change in moving speed of the image surface is determined depending on the interval between times at which the defocus amounts are detected. Therefore, in order to perform precise predictive drive control, the above-mentioned interval is preferably set to be sufficiently short.
However, when the luminance of an object is low, since the received light amount of the charge accumulation type sensor decreases, and the charge accumulation time is prolonged, the detection interval of the defocus amounts is also prolonged. For this reason, when the object speed suddenly changes, it is impossible to measure a correct image surface moving speed, resulting in over-running of the lens by the predictive drive system.
Also, a multi AF area AF camera is known. This camera comprises a plurality of sets of focus detection optical systems and sensors, detects a plurality of defocus amounts by executing focus detection for a plurality of focus detection areas set on a taking field, determines a single final defocus amount based on these defocus amounts, and drives a focusing lens in accordance with the final defocus amount, thereby achieving an in-focus state of a taking optical system.
However, in the above-mentioned multi AF area AF camera, a time required for AF control (to be referred to as an AF time hereinafter) including the charge accumulation time, the charge transfer time, and the focus detection time of the sensors increases since the number of focus detection areas increases, as compared to a camera which adopts a single AF area system, i.e., performs a focusing operation on the basis of the defocus amount on a single focus detection area, thus prolonging the detection interval of the defocus amount. Therefore, for the same reason as in a low-luminance state, when the object speed suddenly changes, a correct image surface moving speed cannot be measured, resulting in over-running of the lens by the predictive drive system.